Steam generator



K. F. NORDLUND March 25, 1952 STEAM GENERATOR 4 Sheets-Sheet 1 Filed May11, 1944 nl H H l l l l l l l l \NVENTonE. Kmrl F lkg Nqrd luncl.

K. F. NORDLUND STEAM GENERATOR March 25, 1952 4 Sheets-Sheet 2 Filed May11, 1944 N YENT Q Kmrl FQU K. F. INORDLUND March 25, 195 2 STEAMGENERATOR 4 Sheets-Sheet 3 Filed May 11, 1944 \NVENTQR. Rad Feflke N o rd\und Patented Mar. 25, 1,952

STEAM GENERATOR Karl Folke Nordlund, Edsviken, Sweden Application May11, 1944, Serial No. 535,068 In Sweden May 14, 1943 11 Claims.

Relatively long tube coils, preferably of small internal diameter, arebeing increasingly employed as steam generating tubes in modern steamgenerators. Their principal advantages are that they take upconsiderably less space, provide a highly effective heating surface, and

finally-particularly at high steam pressuressave a great deal of weightowing to the small tube wall thickness required. Whether the circulationin the coils is forced or natural, the water will not flow steadilythrough them, but will pulsate, causing strong forward flow impulses toalternate with insignificant rates of flow or even retrogression.

These pulsations may in certain circumstances cause circulation troublesand occasionally endanger the safe Working of the plant. If, as isusually the case, a large number of tube coils are connected to a commoninlet header receiving the circulating water from a steam separatorcommon to several inlet headers, the pulsations may cause steam to enterthe inlet headers periodically or accidentally. Although the forwardflow impulses will eventually force that steam back into the steamgenerating tubes, its movements may in the meantime have affected thecirculation process unfavourably.

Should, for instance, steam bubbles enter not only the inlet header butalso the tubes supplying circulating water to that header, any steambubbles which for some reason, in spiteof the forward flow, remain inthe descent tubes may to a greater or less extent hinder the circulatingwaters entering the inlet header, and consequently also the tube coils.Particularly if the descent tubes carry the water to the inlet header bygravity, thus creating a natural circulation in the tube coils, steamentering such descent tubes may endanger the whole circulation, foranatural circulation is produced and maintained by the dilference inspecific gravity of the column of water in the descent tubes and that ofthe column of steam emulsion in the steam generating tubes and theascent tubes. If enough steam should enter the descent tubes, thisdiiference in weight might diminish to insignificance, which wouldnaturally have an adverse effect on the circulation. The result may bethat too little water will reach the tube coils for the plant to beWorked safely.

In the. event of the tube coils being attached to the inlet header atdifferent levels, any steam forced into them by the pulsations may also-sub-- stantially alter the distribution of the circulating water to thedifferent tube coils. The steam will rise to the top of the header andmay even have time to collect there in sufiicient quantity to deplacepart of the water before being again forced into the coils. In any casethe steam will be concentrated to the upper part of the inlet header andthe topmost coils will accordingly be fed not with water alone but witha mixture of Water and steam which may contain so little water that thiswould all hell away before reaching the other ends of the coils wherethe gradually increasing salt deposits might quickly result in tubebreaks.

The problem of preventing these detrimental effects of the pulsatingflow in the steam generating tubes may be solved in two fundamentallydifferent ways. As the trouble is actually due to steam entering theheader serving to distribute the water to the tube coils, the steam caneither be prevented from reaching as far as that header or allowed toenter the header to be removed from there as quickly as possible and insuch a way that it will cause no trouble of the kind described above.

Devices are known which will solve the trouble in the first of theseways. Such are for instance the choke nozzles fixed in the steamgenerating tube inlets of the La Mont forced circulation boilers.Although satisfactory reliability in working has gradually been achievedby this means, it has several attendant drawbacks. For an effectivestabilization of the forward flow in the steam generating tubes, thepressure reduction in these choke nozzles must be of the same order ofmagnitude as the loss of pressure in the tube coils in overcoming theresistance to the flow. The coils must therefore be made substantiallyshorter than if the available pressure could be used solely forovercoming the resistance. Further, there will be a relatively largerisk of these rather small nozzles (down to 4 mm. in diameter) siltingup. To eliminate that risk it has proved necessary to provide eachnozzle with a kind of strainer with a large number of smaller holes thanthat of the nozzle. But even such strainer may silt up, and the descenttube to the inlet header has therefore also had to be provided with alarge strainer which must be inspected occasionally and the silt cleanedout.

The present invention solves the above problem by the second of themethods mentioned above. Any steam entering the inlet header is removedby one or more special escape tubes and passes through them to somesteam carrying part of the steam generator higher than the inlet headerand behind the steam generating tubes in the 3 circulating system, e. g.to the outlet header collecting the steam from the steam generatingtubes or to the ascent or collecting tubes leading the mixture of steamand water from that outlet header to the steam separatororfinally-directly to the steam separator.

The structural details and the manner in which this device functions areindicated in the attached drawings, Figs. 15.

Figs. 1a (vertical view) and 1?) (horizontal section) represent a steamgenerator designed for natural circulation. The steam generating surfaceis heated by convection and consists of the tube coils I placed in ahorizontal gas flue through which the hot gases pass in the direction ofthe arrows. The gases are here supposed to come from some suitablefireplace or furnace, in which latter case the steam generator would bea flue-gas boiler. From the steam separator B the water is taken throughthe descent tube 4 through a water 'seal to the lower end 5 of avertically mounted inlet header 2. From this the water passes into tubecoils I assembled in horizontal layers one above the other; each coil ismade up of a number of straight tubes placed at right angles to thedirection of the gas flow and connected to one another by sections oftubes bent to an angle of 180 to form continuous steam generating tubecoils. The steam generated in these coils then goes to the outlet header3 together with any circulating water that has not yet been vaporized.From the outlet header the mixture of steam and Water passes through theascent tube l2 to the steam separator 6 in which the water is separatedfrom the steam. From the upper end I of the inlet header the previouslymentioned escape tube 8 leads directly to' the steam-containing part ofthe steam separator 6.

Helical or oblique guide vanes are fitted inside the descent tube 4 atthe flange 5 which connects it from below to the inlet header to imparta rotatory movement to the circulating water before it enters the inletheader. These guide vanes 9 may for example be made as shown in Figs. 2a(vertical section) and 2b (horizontal view from above). The vanes arethere made of thin, plain plates set at a certain angle-say 45-andwelded both to the inside of the pipe wall and to a separate small pipe10 for the purpose of giving firmness to the whole guide vanestructure.A similar device may if desired be placed inside the escape tube 8,close to its inlet opening, in order to stop the rotatory movement ofthe water when no longer required. A reduction of the flow resistance inthe escape tube might thus be obtained.

Owing to the great horizontal lengths of heating surface of these tubecoils, strong pulsations will be set up there, and the steam generatedin them will not only flow forward, but will also periodically be flungback towards the inlet header 2. As any steam entering the verticalinlet header will successively rise towards its top, the upper part ofthe header will contain a greater proportion of steam than the lowerwith consequent great risk of too much steam and too little waterentering the highest tube coils. Thanks to the escape tube 8, by whichsuch steam as is not immediately drawn into the tube coils in passingthrough the header will successively escape, this risk is substantiallyreduced. By suitable dimensioning of the guide vanes according to Fig. 2described above, a rotatory movement is imparted to the Water in theinlet header,

which has the effect of collecting practically all the steam that entersthe header in its centre while flinging the water outwards to itsperiphery. The advantage of this is that all the tube coils, in whichthe pressure is lower than in the inlet header, will be filled only ormainly with water and their satisfactory cooling thus secured,irrespective of whether they are at a higher or lower level in theheader. At the same time all the steam collected in the centre of theheader will escape by the outlet pipe 8 to the steam separator 6.

In this way a double circulation system is set up with the descent tube4 common to both. Part of the water flowing through the descent tube 4will enter the tube coils, where a minor portion of it is converted intosteam. The resultant mixture of water and steam flows into the outletheader 3 and from there by the ascent tube l2 to the steam separator 6in which the steam is separated from the water. Another portion of thewater from the descent tube 4 will in the inlet header be mixed withsteam coming back into the header from the tube coils and will leave theheader-still mixed with the steam by the escape tube 8 to the steamseparator 6. Experiments made have proved that it is easy to sodimension the tubes that even if the quantities of steam escapingthrough the escape tube are relatively small, the quantity of watercarried with it may be many times larger than that which flows into thetube coils. This does not mean that the circulation through the escapetube will to any appreciable extent rob the circulation through theheating coils of its water, since the amount of water flowing throughthe descent tube 4 will also be several times larger than it would be ifthere were no escape tube 8. The greater the quantity of steam flowingback into the inlet header, the greater will also-up to a certainmaximum which in practice need not be exceeded-the quantity of watercirculating through the escape tube be.

This circumstance has two exceedingly favourable secondary efiects overand above the solution of the problem which is the object of the presentinvention.

One is that the reaction on the flow of water in the descent tube of anypulsations in the individual steam generating tubes will be considerablyreduced. More especially will a reversal of the direction of the flow inthe descent tube be made more diflicult, and any inflow of steam intoits vertical portion is therefore prevented. This is directly connectedwith the manifold increase of the rate of flow of the water in thedescent tube by the introduction of the escape tube. Substantiallystronger back pressure impulses from the tube coils will therefore berequired for a reversal of the direction of the flow in the descent tubethan was the case before the escape tube was introduced. This providesthe requisite safety margin to prevent any disturbances of thecirculation of this kind. That safety margin may be said to consist inthe aggregate kinetic energy of the water flowing through both thedescent and escape tubes.

The other favourable secondary effect consists in a substantial increasein the ability of the generator to withstand, without failure, thestresses of sudden pressure reductions due to sudden increases in thequantity of steam taken out of it. This ability mainly depends on therate of flow of the water in the descent tubes. The risk of failure liesin that the water in the feed pipes may begin to boil when the pressuresuddenly drops, which might prevent the circulation. But if the pressuredrop, which is initiated in the steam separator, is not too rapid, thedescent tube will be fed continuously with water which owing to thepressure drop has given off steam in the steam separator and hasaccordingly assumed the lower temperature corresponding to the lowerpressure. This water will flow towards parts where the pressure ishigher and its vaporization will therefore cease. On account of thehydrostatic pressure difference, the water in the lower parts of thedescent tubes is subjected to higher pressure than the water in thesteam separator. The faster the downward flow of water in the descenttube, the quicker can the pressure in the steam separator be reducedwithout danger of vaporization in the descent tube. The limit is reachedwhen the increase of pressure due to the greater depth of the waterbelow its free surface in the steam generator balances the continuousriction of the steam pressure in the steam separator.

As the backward flow of steam into the inlet header and escape tube willnot substantially increase the flow of water in the descent tube, thepresent invention will automatically also increase the ability of thesteam generator to withstand rapid reductions of the steam pressure.What has now been said applies to heavily loaded steam genefators, foronly when much heat is supplied to the tube coils will the pulsations inthese be strong enough to force the steam back periodically to the inletheader. They will not be able to do so when the heat load is low, andthere will consequently be no flow of water through the escape tube. Arapid lowering of the pressure will then cause the water to boil soonerin the escape tube than in the descent tube, which is of course flowingdownwards, i. e. towards levels of higher pressure. This will produce apreviously non-existing difference of weight between the water column inthe descent tube and that in the escape Ytube, causing the steamemulsion in the escape tube to begin to flow rapidly upwards, increasingthe rate of flow of the water in the descent tube proportionately. Thisprovides increased security against boiling in the descent tube, andaccordingly also against disturbances of the circulation. vaporizationin the escape tube will increase with falling steam pressure, whichaccordingl also applies to the rate of flow in the descent tube. Themargin of safety as regards disturbances of the circulation will thusincrease with the risk of such disturbances, i. e. with the rate atwhich the pressure drops. The escape tube thus provides increasedsecurity against disturbances of circulation connected with any suddenreduction of pressure in the steam genorator, whether the pressure inthe generator is high or low.

Fig. 3 shows, inside the dot-and-dash line, how the tube coils arearranged on the wall of a steam boiler furnace to form what is called acooling wall, also servingto cool the brickwork and protect this fromthe intense heat radiated by the furnace flames. The same figures areused as in Fig. 1, and tube coils, descent tube, inlet header, outletheader, ascent tube, and escape tube may be easily recognized there. Theessential difference is that the inlet header 2 is here horizontal. Itmight then be asked how it is that the periodical return flow of steaminto the inlet header in this case will really pass out by the escapetube 8 and not by'the descent tube 4, seeing that both these tubes areconnected to the inlet header at exactly the same level and that thesteam ought therefore to flow into the one tube as easily as into theother. To make perfectly certain of the tubes functioning in the desiredmanner, it is accordingly necessary to obstruct the entry of steam intothe descent tube and to make it definitely more easy for the steam toenter the escape tube than the descent tube. This obstructive efiectisachieved by the arrangement shown in Fig. 3 where the pipe is carriedhorizontally a not too short distance reckoned from the inlet headerbefore it is bent upward, while the escape tube turns vertically upwardsimmediately on leaving the inlet header. This difference in thearrangement of the tubes, although not making the actual entry of thesteam into tube 4 more difficult than into tube 8, will certainlyprevent the steam reaching the vertical portion of the descent tubebefore sufiicient steam has reached the vertical portion of the escapetube to make the difference in the specific gravities of the watercolumn in the vertical portion of the descent tube on the one hand andthe column of steam emulsion in the escape tube on the other largeenough to set up a strong circulation which will compel the water toflow down the descent tube into the inlet header, and from this to theescape tube. The steam is thus prevented by the flow of the water in theopposite direction from passing through the descent tube instead ofthrough the escape tube. The escape tube is not here connected directlyto the steam separator, but to the outlet header, which with correctlydimensioned pipes is also possible.

Fig. 4a is a section of a vertical flue, in which the hot gases fromsome kind of furnace rise from below upwards; giving off their heat to anumber of steam-generating tube coils. These coils are connected to ahorizontal inlet header visible in section on the right of the drawing.The coils are taken direct to the steam separator 6. Fig. 4b shows thesteam generator seen in Fig. 4a from the left, and in particular permitsa study of the arrangement of the tubes at the inlet header. The waterreaches the inlet header from the steam separator 6 through the descenttube ti whichbefore entering the header 2- is bent to form a trap l3 toprevent any steam that might flow back to the inlet header finding itsway through the vertical portion of the descent tube. The steam willtherefore pass through the escape tube 8 direct to the steam separator6. Here it is thus the trap [3 that obstructs any return flow of steaminto the vertical portion of the descent tube, and so secures the;functioning of the escape tube as an escape tube, and of the descenttube as a-feed pipe, although they both connect the steam separator tothe inlet header and are joined to the latter at the same level. As thetube coils l are connected to the lower side of the inlet header, anysteam flowing back into this from the tube coils will owing to its lowerspecific gravity, collect mainly at its top, and so be prevented fromagain entering the coils at the times when there is a forward flow intothe openings to the tube coils. Water will thus always be supplied tothe tube coils even though these may periodically discharge steam intothe inlet header. An equal distribution of the circulating water to theseveral tube coils and accordingly also the satisfactory cooling of thecoils while working are secured by this means.

r The methodmentioned above of attaching the tube coils to the lowerpart of a horizontal inlet header is intended to be used also in thearrangement according to Fig. 3, although this is not clearly marked inthe drawing.

Fig. a shows a longitudinal section of the heating surface in ahorizontal flue, consisting of pendant tube coils I made up exclusivelyof vertical tube portions connected by sections bent to an angle of 180.Fig. 5b is a cross section of the same flue. The bottom tube coil is metby gas screens 14, between which the ashes fall down and can be rakedout through the hand holes 11. These screens prevent the hot gasespassing by the pendant convection surfaces. The ends of the coils areconnected to two horizontal headers, the inlet header 2 and the outletheader 3. The inlet header is connected to the water-filled part of thesteam separator 6. The other end of the inlet header is connected to thesteam-filled part of the steam separator 6 by the escape tube 8, whichhas exactly the same function here as in previously describedarrangements, i. e. to return any steam flowing back into the inletheader from the tube coils. The outlet header 3 is connected to thesteam-filled part of the steam separator 6. In order to compel thedescent tube to function as a feed pipe and the escape tube as an escapetube, the former is provided with a trap l3 immediately before enteringthe inlet header, thus preventing the back flow of steam from enteringthe vertical portion of the descent tube.

The water will circulate from the inlet header to the outlet header ifonly the steam separator 6 is placed at a sufiicient height above theheating surface. The force producing this circulation is the differencein specific gravity of the column of water in the descent tube and thecolumn of steam emulsion in the ascent tube I2 from the outlet header tothe steam-filled part of the steam separator 6. A sine qua non of thatcirculation is thus that there must be only water in the descent tube 4and a mixture of steam and water, rich in steam, in the ascent tube I2.That condition is continuously maintained by the arrangement described,for the descent tube 4 is the only one of the three tubes 4, 8, and i2which leaves the steam sepaartor at a point below its free water surfaceand is therefore capable of conveying water to the steamgeneratingsurface in the flue, and at the same time the only one of the said threepipes in which steam from the steam-generating surface cannot enter aslong as the escape tube 8 provides a free passage for any steam that mayflow back into the inlet header. Incidentally, the return flow of steamcan be substantially reduced by letting the water enter the tube coilsat the ends situated where the flue gases, after having been cooled bythe heating surface, pass beyond this. Less steam will therefore begenerated there than at the other end of the heating surface, where thetube coils are connected to the outlet header where they encounter thehot flue gases before these are cooled. It is obviously not essentialfor the obstacle preventing the return flow of steam from entering theescape tube and not the descent tube to be a water trap as figured inFig. 5b. The main thing is that the obstacle employed is sufficient tomake descent tube and escape tube function as desired.

The said obstacle may for example be a choke nozzle which will coke offthe pressure in the descent tube to a certain extent whenever the waterflows towards the inlet header. In order to enter the descent tube thesteam must accordingly overcome that pressure, which it can never do inthe inlet header provided the escape tube is sufficiently wide indiameter to allow the backflow of steam to escape to the steam separatorwithout increasing the pressure too much. The effect of such a chokenozzle will obviously be greater if it is made as shown in Figs. 6a and6b. The opening IS in the nozzle plate i5 is here located in itslowermost part, and sufficient steam must therefore accumulate in theinlet header 2 to depress the water to the top of the opening l6 beforesteam can force its way into the descent tube 4. If the escape tube 8 issufficiently large this will be impossible, and an obstacle of thissimple nature may therefore be quite sufficient to achieve the desiredeffect. The nozzle design may of course be used not only in thearrangement shown in Figs. 5a and 5b but also in all cases in which thetube coils come from horizontal inlet headers, thus also in thearrangements shown in Figs. 3 and 4.

When the steam generating tubes carry high heat loads the pulsationsbecome very brisk and powerful, and large masses of steam will thenperiodically flow backwards towards the inlet header and the escapetube. The internal diameters of these must therefore be fairly large soas to give sufficient room for these quantities of steam. With a view tokeepi g down the dimensions of the inlet header and the escape tube.however, an effort to reduce the amount of backflowing steam may bemade. The previously mentioned, well-known method of placing chokenozzles in the openings of the steam generating tubes, which is employedin for example the La Mont boiler, may be used for that purpose. Thedrawbacks of thisthe risk of the nozzles silting up and of a largeportion of the available pressure being consumed in the chokingprocessmay be avoided by making the nozzles large enough to avoid anyrisk of their silting up. At that sizee. g. when the nozzle orifice isnot less than 12 to 15 mm. in diameteronly an insignificant portion ofthe available pressure will be lost, and at least of it may as a rule beused for overcoming the flow resistance in the coils, and so make theemployment of long coils practicable. Although it is of courseimpossible entirely to prevent a back-flow of steam into the inletheader by this means, the amount of such back-flow will be reduced to afraction of the value it would otherwise attain, and the dimensions ofinlet header and escape tube may therefore be substantially reducedwithout any risks of the circulation being disturbed.

The choke nozzles may for instance be constructed as shown in Fig. 7,where 3 is the inlet opening of the tube coil, which is attached to thewall of the inlet header 2. The choke nozzle I consists of an exteriorlyslightly conical plug driven into the inlet opening of the tube coilwhich has previously been reamered to the same angle as the chokenozzle. Centrally in this cone is drilled a cylindrical hole of suitablediameter.

The devices now described, and the invention as a whole, are naturallynot limited to the arrangements of the tubes shown in Figs. 1-6, but isequally applicable to all steam generators characterized by a number ofparallel-coupled tube coils for steam generation of any form or shape,or to steam generating tubes of other designs connected to anarbitrarily situated inlet header.

What I claim is:

l. A steam generator for natural circulation comprising a plurality ofparallel coupled steam generating tubes formed by tube coils of smallinternal diameter and great length; an elongated inlet header common tothe inlet ends of said steam generating tubes, a steam separator abovesaid inlet header and tubes; connections between said steam separatorand said inlet header and steam generating coils arranged to form aclosed circuit for water and generated steam with defined direction offlow in the steam generating tubes directed from said inlet header toall of said tubes; the connections between said inlet header and saidsteam separator consisting of at least one descent tube arranged for anuninterrupted flow of the circulation water from the water space of thesteam separator by gravity to one end of the inlet header, the lowestpoint of said descent tube being disposed at least as low as the lowerpart of the inlet header, and at least one steam escape tube connectedto the inlet header at the opposite end thereof above the connectionpoints of the steam generating tubes therewith and to the steamseparator above the level of the water therein and further being sodimensioned and arranged that it is able quickly to lead ofi from theupper portion of the inlet header to the steam space of the steamseparator the steam quantities whichnow and then on account ofirregularities in the circulation enter the inlet header and thusprevent these steam quantities from disturbing the circulation processin the circuit as a whole.

2. A steam generator for natural circulation comprising a plurality ofparallel-coupled steam generating tubes formed by tube coils of smallinternal diameter and great length; an elongated inlet header common tothe inlet ends of said steam generating tubes; a steam separator abovesaid inlet header and tubes; connections between said steam separatorand said inlet header and steam generating coils arranged to form aclosed circuit for water and generated steam with defined direction offlow in the steam generating tubes directed from said inlet header toall of said tubes, the inlet openings of the steam generating tubesbeing at least so great that at the temporary occurring tendency of thesteam-water mixture to flow back to the inlet header, not only water butalso a part of the enerated steam is allowed to enter said inlet header;the connections between said inlet header and said steam spaceconsisting of at least one descent tube arranged for an uninterruptedflow of the circulation water from the water space ofthe steam separatorby gravity to one end of the inlet header, the lowest point of saiddescent tube being disposed at least as low as the lower part of theinlet header, and at least one steam escape tube connected to the inletheader at the opposite end thereof above the connection points of thesteam generating tubes therewith and to the steam separator above thelevel of the water therein and further being so dimensioned and arrangedthat it is able quickly to lead oiT from the upper portion of the inletheader to the steam space of the steam separator the steam quantitieswhich new and then on account of irregularities in the circulation enterthe inlet header and thus prevent these steam quantities from disturbingthe circulation process in the circuit as a whole.

3. A steam generator for natural circulation comprising a plurality ofparallel-coupled steam generating tubes formed by tube coils of smallinternal diameter and great length; an elongated inlet header common tothe inlet ends of said steam generating tubes; a steam separator abovesaid inlet header and tubes; connections between said steam separatorand said inlet header and steam "generating coils arranged to form aclosed circuit for water and generated steam with defined direction offlow in the steam generating tubes directed from said inlet header toall of said tubes, the inlet openings of the steam generating tubesbeing at least so great that at the temporary occurring tendency of thesteam water mixture to now back to the inlet header, not only water, butalso a part of the generated steam is allowed to enter said inletheader; the connections between'said inlet head'- er and said steamspace consisting of at least one descent tube arranged for anuninterrupted flow of the circulation water from the water space of thesteam separator by gravity to one end of the inlet header, the lowestpoint of said descent tube being disposed at least as low as the lowerpart of the inlet header, and at least one steam escape tube connectedto the inlet header at the opposite end thereof above the connectionpoints of the steam generating tubes therewith and to the steamseparator above the level of the water therein and further being sodimensioned and arranged that it is able quickly to lead off from theupper portion of the inlet header to the steam space of the steamseparator the steam quantities which now and then on account ofirregularities in the circulation enter the inlet header and thusprevent these steam quantities from disturbing the circulation processin the circuit as a whole, this function of the escape tube beingsupported by obstruction means to the back-flowing steam in the descenttube and always permitting the water from the water space of the steamseparator to pass continually in the defined direction of circulation,said escape tube being free of obstruction, whereby the back-flowingsteam is forced to enter the escape tube in preference to the descenttube in esca ing from the inlet header to the steam separator.

4, steam generator as set forth in claim 3 in which the obstruction tothe backflow of steam in the descent tube consists of a water sealarrangement in which the water passing to the inlet header is forced topass a lower level than the level of the lowermost of the inlet ends ofthe steam generating tubes.

5. A steam generator as set forth in claim 3 in which the obstruction tothe backflow of steam in the descent tube consists of an elon: atedsubstantially horizontal section of said tube on a level not essentiallyhigher than the lower part of the inlet header.

6. A steam generator as set forth in claim 3 in which the obstruction tothe backflow of steam in the descent tube comprises choking means in thedescent tube. I

'7. A steam generator for natural circulation comprising a plurality ofparallel-coupled steam generating tubes formed by tube coils of smallinternal diameter and great length; a vertical inlet header common tothe inlet ends of said steam generating tubes; a steam separator abovesaid: inlet header and tubes; connections between said steam separatorand said inlet header and steam generating coils arranged to form aclosed circuit for water and generated steam with defined direction offlow in the steam generating tubes directed from said inlet header toall of said tubes, the inlet openings of the steam generating tubesbeing at least so great that at the temporary occurring tendency of thesteam-water mixture to flow back to the inlet header, not only water,but also a part of the generated steam is allowed to enter said inletheader; the connections between said inlet header and said steam spaceconsisting of at least one descent tube arranged for an uninterruptedflow of the circulation water from the water space of the steamseparator by gravity to the inlet header, and connected to the lower endof the latter, and at least one steam escape tube connected to the inletheader at the upper end thereof above the connection points of the steamgenerating tubes therewith and to the steam separator above the level ofthe water therein and further being so dimensioned and arranged that itis able quickly to lead off from the upper portion of the inlet headerto the steam space of the steam separator the steam quantities which nowand then on account of irregularities in the circulation enter the inletheader and thus prevent these steam quantities from disturbing thecirculation process in the circuit as a whole.

8. A steam generator as set forth in claim '7 including means forefiecting a rotary movement of the water about the vertical axis of theinlet header, said means being situated at the lower end of the saidheader, in order to get the now and then back-flowing steam collected inthe center of the header and prevent it from re-entering the inlet endsof the steam generating tubes, thus securing an equal distribution ofwater in the said tubes.

9. A steam generator, as set forth in claim 2, in which the inlet headerextends horizontally, the inlet ends of the steam generating tubes beingsituated in the lower portion of the said header, so that thebaekfiowing steam will be collected in the upper part of the inletheader and from there lead to the escape tube, without having apossibility of re-entering the steam generating tubes.

10. A steam generator for natural circulation comprising a plurality ofparallel-coupled steam generating tubes formed by tube coils of smallinternal diameter and great length, each of them made of a straighttube, which is bent back in at least two points, so that said tube willcontain several straight tube portions of different direction, at leasttwo of them together with the tube bend between them being situated in ahorizontal plane; an elongated inlet header common to the inlet ends ofsaid steam generating tubes; a steam separator above said inlet headerand tubes; connections between said steam separator and said inletheader and steam generating coils arranged to form a closed circuit forwater and generated steam with defined direction of flow in the steamgenerating tubes directed from said inlet header to all of said tubes,the inlet openings of the steam generating tubes being at least so greatthat at the temporary occurring tendency of the steam-water mixture toflow back to the inlet header, not only water, but also a part of thegenerated steam is allowed to enter said inlet header; the connectionsbetween said inlet header and said steam space consisting of at leastone descent tube arranged for an uninterrupted flow of the circulationwater from the water space of the steam separator by gravity to one endof the inlet header, the lowest point of said descent tube beingdisposed at least as low as the lower part of the inlet header, and atleast one steam escape tube connected to the inlet header at theopposite end thereof above the connection points of the steam generatingtubes therewith and to the steam separator above the level of the watertherein and further being so dimensioned and arranged that it is ablequickly to lead off from the upper portion of the inlet header to thesteam space of the steam separator the steam quantities which now andthen on account of irregularities in the circulation enter the inletheader and thus prevent these steam quantities from disturbing thecirculation process in the circuit as a whole.

11. A steam generator for natural circulation comprising a plurality ofparallel-coupled steam generating tubes formed by tube coils of smallinternal diameter and great length, each of them made of a straighttube, which is bent back in at least two points, so that said tube willcontain several straight tube portions of vertical direction; anelongated inlet header common to the inlet ends of said steam generatingtubes; a steam separator above said inlet header and tubes; connectionsbetween said steam separator and said inlet header and steam generatingcoils arranged to form a closed circuit for water and generated steamwith defined direction of flow, in the steam generating tubes directedfrom said inlet header to all of said tubes, the inlet openings of thesteam generating tubes being at least so great that at the temporaryoccurring tendency of the steam-water mixture to flow back to the inletheader, not only water but also a part of the generated steam is allowedto enter said inlet header; the connections between said inlet headerand said steam space consisting of at least one descent tube arrangedfor an uninterrupted flow of the circulation water from the water spaceof the steam separator by gravity to one end of the inlet header, thelowest point of said descent tube being disposed at least as low as thelower part of the inlet header, and at least one steam escape tubeconnected to the inlet header at the opposite end thereof above theconnection points of the steam generating tubes therewith and to thesteam separator above the level of the water therein and further beingso dimensioned and arranged that it is able quickly to lead off from theupper portion of the inlet header to the steam space of the steamseparator the steam quantities which now and then on account ofirregularities in the circulation enter the inlet header and thusprevent these steam quantities from disturbing the circulation processin the circuit as a whole.

KARL FOLKE NORDLUND.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 592,891 Solignac Nov. 2, 18971,874,552 Langvand Aug. 30, 1932 1,950,454 Lucke Mar. 13, 1934 FOREIGNPATENTS Number Country Date 143,175 Austria Oct. 25, 1935 512,049 GreatBritain Nov. 24. 1936

